This disclosure relates generally to an improved way to pour molten metal used in a semi-permanent mold casting operation, and more particularly to improving riser feedability to reduce defects in the casting upon cooling.
Casting is a metal shaping process characterized by pouring a molten metal into a mold and allowing it to solidify. One of the advantages of metal casting is that the resulting product can virtually have any configuration. Although casting offers many benefits, it suffers from difficulties in quality of the finished cast articles due to incidents of formation of a shrinkage cavity (hot spot), cold shuts, or misrun. Most metals are less dense as a liquid than as a solid so castings naturally shrink upon cooling. The natural shrinkage of liquid metal during solidification can leave a void at the last point to solidify called a shrinkage cavity.
To reduce the occurrence of shrinkage cavity formation, casting systems frequently include one or more risers. A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Excess molten metal flows into the riser during mold filling. The additional molten metal is needed to compensate for contractions or shrinkage of the molten metal, which occur during the casting process. Metal from the riser fills such voids created in the casting when metal from the casting contracts. In order to fill voids left from metal contraction, the metal from the riser must remain in a liquid state for a longer period of time than the bulk casting. An optimal design of riser will help in reducing shrinkage cavities by ensuring that molten metal can readily flow into the casting when the need arises.
However, risers are only effective if three conditions are met: the riser cools after the casting, the riser has enough material to compensate for the casting shrinkage, and the casting directionally solidifies towards the riser. For the riser to cool after the casting, the riser must cool more slowly than the casting. In current production of semi-permanent mold casting of cylinder heads for example, a bottom fill or a side fill gating design and a heavy open riser are typically used. During mold fill, the molten metal flows through the casting cavity with the riser being the last section to fill. As a result, the temperature of the molten metal in the riser is generally low and riser feedability is significantly reduced because the molten metal losses heat when it passes through the casting cavity.
There is a continuing need for a method of improving riser feedability to allow for improved back filling of the mold cavity with molten metal to prevent shrinkage cavity formation during solidification and associated shrinkage of the bulk metal in the casting cavity.